In late years, it has been proposed to construct an acceleration sensor or an angular velocity sensor as a micromachine using semiconductor manufacturing technologies. This type of micromachine requires a mass body having a relatively large inertial mass. As a method of forming such a mass body, it is conceivable to form a through-hole (including a slit-shaped through-hole) in a semiconductor substrate having a relatively large thickness dimension (several hundred μm) so as to separate a mass body from the remaining region. The through-hole is formed in a semiconductor substrate with a relatively large thickness dimension by means of an etching technique, such as a wet etching process or a reactive-ion etching process. This technique is disclosed, for example, in Sunil A. Bhave et al. “AN INTEGRATED VERTICAL-DRIVE, IN-PLANE-SENSE MICROGYROSCOPE”, TRANSDUCERS '03 (IEEE, The 12th International Conference on Solid-State Sensor, Actuators and Microsystems), USA, Jun. 8-12, 2003, p. 171.
It is known that, in the process for forming a through-hole in a semiconductor substrate using the above technique, the speed or rate of removing a semiconductor material is dependent on the width of an opening provided in a mask. That is, a region of the semiconductor substrate corresponding to the relatively wide opening width has a larger removal rate in the depth direction as compared to a region corresponding to the relatively narrow opening width. Thus, in a process of forming two types of through-holes different in width dimension in a common semiconductor substrate, it is likely that, at the time when one through-hole having a larger width dimension penetrates through the semiconductor substrate, the other through-hole having a smaller width dimension has not yet penetrated through the semiconductor substrate. If the difference in the opening width is small, the difference, between respective times necessary for the wide-width through-hole to penetrate through the semiconductor substrate and necessary for the narrow-width through-hole to penetrate through the semiconductor substrate will fall within an allowable error or tolerance. However, if the difference in the opening width is large, it must be required to wait for a relatively long time until the narrow-width through-hole penetrates through the semiconductor substrate after the wide-width through-hole penetrates through the semiconductor substrate. This causes the problem that the inner peripheral surface of the wide-width through-hole is eaten away or corroded during the waiting time, resulting in deteriorated dimensional accuracy of the wide-width through-hole.
If it is attempted to avoid this kind of problem by use of conventional techniques, the process of forming two types of through-holes different in width dimension has to be divided into two separate processes. This causes the increase in process time for forming the through-holes, and in the number of processes due to the need for protecting one through-hole formed in a preceding process by a protective material, and removing the protective material. Consequently, the process time will be considerably increased.